CHAPTER 13 How Populations Evolve Figures 13.1 13.5 PowerPoint Lecture Slides for Essential Biology, Second Edition & Essential Biology with Physiology Presentation prepared by Chris C. Romero Neil Campbell, Jane Reece, and Eric Simon All humans are connected by descent from African ancestors The same types of bones make up the forelimbs of humans, cats, whales, and bats BIOLOGY AND SOCIETY: PERSISTENT PESTS Mosquitoes and malaria In the 1960s, the World Health Organization (WHO) began a campaign to eradicate the mosquitoes that transmit malaria It used DDT, to which some mosquitoes have evolved resistance 1
Insecticide application Chromosome with gene conferring resistance to insecticide Survivors Additional applications of the same insecticide will be less effective, and the frequency of resistant insects in the population will grow Figure 13.1 CHARLES DARWIN AND THE ORIGIN OF SPECIES Charles Darwin s On the Origin of Species by Means of Natural Selection Was published on November 24, 1859 Focused biologists attention on the great diversity of organisms The basic idea of natural selection is that Organisms can change over generations Individuals with certain heritable traits leave more offspring than others. 2
The result of natural selection is evolutionary adaptation. (b) A Trinidad tree mantid that mimics dead leaves (a) A flower mantid in Malaysia (c) A leaf mantid in Costa Rica Figure 13.2 Darwin s Cultural and Scientific Context The Origin of Species Challenged the notion that the Earth was relatively young and populated by unrelated species. The Idea of Fixed Species The Greek philosopher Aristotle held the belief that species are fixed and do not evolve The Judeo-Christian culture fortified this idea and suggested that the Earth is only about 6,000 years old. 3
Lamarck and Adaptive Evolution In the mid-1700s, the study of fossils began to take form as a branch of science Naturalist Georges Buffon Suggested that the Earth might be older than 6,000 years Observed similarities between fossils and living species. Jean Baptiste Lamarck Suggested that organisms evolved by the process of adaptation Also suggested some erroneous ideas, such as the inheritance of acquired characteristics. The Voyage of the Beagle In December 1831, Darwin left Great Britain on the HMS Beagle to explore the world. PACIFIC OCEAN North America Galápagos Islands ATLANTIC OCEAN South America Great Britain Europe Africa Equator Andes Cape of Good Hope Australia Cape Horn Tierra del Fuego Tasmania New Zealand Figure 13.3 4
The Galapagos Islands are located about 900 Km off the west coast of Ecuador. On his journey on the Beagle, Darwin Collected thousands of specimens Observed various adaptations in organisms. Darwin was intrigued by The distribution of organisms on the Galápagos Islands The fact that Galápagos organisms resembled those in South America. Figure 13.4 5
The New Geology Darwin was strongly influenced by the writings of geologist Charles Lyell Darwin reasoned that The Earth is very old and has been shaped by slow processes Subtle processes occurring over long periods of time can cause great change. Descent with Modification Darwin made two main points in The Origin of Species Organisms inhabiting Earth today descended from ancestral species Natural selection was the mechanism for descent with modification. Elephas maximus (Asian elephant) Loxodonta africana (African savannah elephant) Loxodonta cyclotis (African forest elephant) Today 10,000 years ago 2 million years ago 5.5 million years ago Mammut (mastodon) Stegodon Deinotherium Platybelodon Mammuthus (mammoth) 24 million years ago Moeritherium Barytherium 34 million years ago Figure 13.5 6
EVIDENCE OF EVOLUTION Biological evolution has left marks of evidence. The Fossil Record Fossils Are preserved remnants or impressions left by organisms that lived in the past Are often found in sedimentary rocks. 1 Rivers bring sediment to the ocean. Sedimentary rocks containing fossils form on the ocean floor. 2 Over time, additional strata are added, containing fossils from each time period. 3 As sea levels change and the seafloor is pushed upward, sedimentary rocks are exposed. Erosion by rivers reveals strata; deeper strata contain older fossils. Younger stratum with more recent fossils Older stratum with older fossils Figure 13.6 7
The fossil record Is the chronology of fossil appearances in rock layers Testifies that organisms have appeared in a historical sequence Fits with other evidence of evolution. Figure 13.7 Paleontologists Are scientists that study fossils Have discovered many transitional forms that link past and present. Figure 13.8 8
Biogeography Biogeography Is the study of the geographic distribution of species First suggested to Darwin that today s organisms evolved from ancestral forms. Many examples from biogeography support evolutionary theory. Australia Koala Kangaroo Figure 13.9 9
Comparative Anatomy Comparative anatomy Is the comparison of body structure between different species Confirms that evolution is a remodeling process. Homology Is the similarity in structures due to common ancestry p. 252 See Fig. 13.10 for example of homologous structures of diverse mammals forelimbs. Human Cat Whale Bat Figure 13.10 10
Comparative Embryology Comparative Embryology is the comparison of structures that appear during the development of different organisms Comparative embryology of vertebrates supports evolutionary theory. Gill pouches Post-anal tail (a) Chick embryo (b) Human embryo Figure 13.11 Molecular Biology Evolutionary relationships among species Leave signs in DNA and proteins Can be determined by comparing genes and proteins of different organisms. 11
Old World monkey Gibbon Orangutan Gorilla Human Chimpanzee Figure 13.12 NATURAL SELECTION AND ADAPTIVE EVOLUTION Darwin s finches Are an excellent example of natural selection and adaptive evolution. (a) Large ground finch (b) Small tree finch (c) Woodpecker finch Figure 13.13 Darwin s Theory of Natural Selection Darwin based his theory of natural selection on two key observations. 12
Observation 1: Overproduction All species tend to produce excessive numbers This leads to a struggle for existence. Figure 13.14 Observation 2: Individual variation Variation exists among individuals in a population Much of this variation is heritable. Figure 13.15 Inference: Differential reproductive success (natural selection) Those individuals with traits best suited for the local environment leave more fertile offspring. 13
Natural Selection in Action Examples of natural selection include Pesticide resistance in insects The development of antibiotic-resistant bacteria Drug-resistant strains of HIV, the virus that causes AIDS. THE MODERN SYNTHESIS: DARWINISM MEETS GENETICS The modern synthesis is the fusion of genetics with evolutionary biology. Populations as the Units of Evolution A population Is a group of individuals of the same species living in the same area at the same time Is the smallest biological unit that can evolve. Figure 13.16 14
Population genetics Focuses on populations as the evolutionary units Tracks the genetic makeup of populations over time. Genetic Variation in Populations Individual variation abounds in populations Not all of this variation is heritable Only the genetic component of variation is relevant to natural selection. A population is said to be polymorphic for a characteristic if two or more morphs, or forms, are present in noticeable numbers. Figure 13.17 15
Sources of Genetic Variation Mutations and sexual recombination Produce genetic variation. Mutations Are changes in the DNA of an organism, is the ultimate source of all variation; most mutations are likely harmful. Sexual recombination Shuffles alleles during meiosis. Analyzing Gene Pools The gene pool Consists of all alleles of all individuals making up a population. 16
Alleles in a gene pool Occur in certain frequencies Can be symbolized by p for the relative frequency of the dominant allele in the population and q for the frequency of the recessive allele in the population. Genotype frequencies Can be calculated from allele frequencies Are symbolized by the expressions p 2, 2pq, and q 2 Allele Frequencies p = 0.8 (R) q = 0.2 (r) Sperm q = 0.2 r p = 0.8 R rr qp = 0.16 RR p 2 = 0.64 R p = 0.8 Rr pq = 0.16 r Eggs q = 0.2 rr q 2 = 0.04 Genotype frequencies p 2 = 0.64 (RR) 2pq = 0.32 (Rr) q 2 = 0.04 (rr) Figure 13.18 17
The Hardy-Weinberg formula Is a mathematical representation of a gene pool Adds up all of the genotypes in a population. Population Genetics and Health Science The Hardy-Weinberg formula can be used to calculate the percentage of a human population that carries the allele for a particular inherited disease. Figure 13.19 Microevolution as Change in a Gene Pool Hardy-Weinberg equilibrium Describes a nonevolving population that is in genetic equilibrium Can be used to determine whether a population is evolving. 18
MECHANISMS OF MICROEVOLUTION Four causes of microevolution are Genetic drift Gene flow Mutations Natural selection Genetic Drift Genetic drift Is a change in the gene pool of a small population due to chance. Only 5 of 10 plants leave offspring Only 2 of 10 plants leave offspring Generation 1 p (frequency of R) = 0.7 q (frequency of r) = 0.3 Generation 2 p = 0.5 q = 0.5 Generation 3 p = 1.0 q = 0.0 Figure 13.20 19
The Bottleneck Effect The bottleneck effect Is an example of genetic drift Results from a drastic reduction in population size. Original population Bottlenecking event Surviving population Figure 13.21 Bottlenecking in a population reduces genetic variation. Some alleles are likely to be lost from the population. Figure 13.22 The Founder Effect The Founder Effect, p.262 Is genetic drift in a new colony ex.galapagos finches 20
Genetic Drift and Hereditary Disorders in Human Populations The founder effect Explains the relatively high frequency of certain inherited disorders among some populations. Figure 13.23 Gene Flow Gene flow Is genetic exchange with another population Tends to reduce genetic differences between populations. Figure 13.24 Mutations Mutations Are changes in an organism s DNA Alone do not have much effect on a large population Can have significant cumulative effects on a population Over the long term, mutation is, in itself, very important to evolution because it is the original source of the genetic variation that serves as raw material for natural selection. p. 263 21
Natural Selection: A Closer Look Of all causes of microevolution, only natural selection is generally adaptive pp263-4. Darwinian Fitness Darwinian fitness Is the contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals. Figure 13.25 Three General Outcomes of Natural Selection Directional selection Shifts the phenotypic curve of a population Selects in favor of some extreme phenotype. Original population Evolved population (a) Directional selection Phenotypes (fur color) Original population Figure 13.26a 22
Diversifying selection Can lead to a balance between two or more contrasting morphs in a population. Original population Phenotypes (fur color) (b) Diversifying selection Figure 13.26b Stabilizing selection Maintains variation for a particular trait within a narrow range. Phenotypes (fur color) Original population (c) Stabilizing selection Figure 13.26c EVOLUTION CONNECTION: POPULATION GENETICS OF THE SICKLE-CELL ALLELE Sickle-cell disease Affects about one out of every 500 African Americans Is more common among African Americans; but why? Figure 13.27 (inset) 23
The sickle-cell allele Confers resistance to the disease malaria Is adaptive in the African tropics where malaria is common. This is stabilizing selection, p.267 Figure 13.27 Survival to sexual maturity, of course, is prerequisite to reproductive success... Production of fertile offspring is the only score that counts in natural selection. p.264 SUMMARY OF KEY CONCEPTS Darwin s Theory of Natural Selection Observations Overproduction of offspring Inference Natural selection: Differential reproductive success Individual variation Visual Summary 13.1 24
Natural Selection: A Closer Look Original population Evolved population Pressure of natural selection Directional selection Diversifying selection Stabilizing selection Visual Summary 13.2 25